Pallet trucks are commonly used to load and unload pallets from a truck. A typical pallet includes a bottom deck, a top deck and support blocks. The support blocks are coupled between the bottom and top decks and define an opening therebetween for receiving lifting members of material handling equipment, such as forks from a pallet truck.
A typical pallet truck includes a cab, a vertical load plate on a front end of the cab, and a pair of forks or tines extending outwards from a lower end of the vertical load plate in a substantially horizontal direction. A load wheel is in each fork. The load wheel may also be referred to as a lift wheel. Each load wheel is coupled to a respective load wheel linkage that is pivotally coupled to the fork. The maximum lift of the forks may vary depending on the pallet truck, but a typical lift height is about 6 inches.
When an operator of the pallet truck extends the forks into an opening of a pallet, the operator cannot readily observe the position of the load wheels because of the boards in the top deck of the pallet and any load carried on the top deck. If the load wheels come to rest on a bottom deck board of the pallet, then downward movement of the load wheels to lift the forks results in separation of the bottom deck board from the pallet.
One approach to this problem is to mount light sensors on at least one of the forks, as disclosed in U.S. Pat. No. 4,747,610. The light sensors detect the positions of the load wheels with respect to the bottom deck boards of the pallet. A disadvantage of light sensors is that they may fail to properly operate due to blockage of the light beams by dirt and debris.